Solutions for Longer Lasting Wood Protection

Introduction

Waterborne semi-transparent exterior wood stains are frequently used to protect and add to the natural beauty of a broad range of coated wood species. As these lightly pigmented stains are usually applied in thin coats, their durability under adverse weather conditions is critical. Evaluation of coating degradation caused by natural weathering is addressed by the European Standard EN 927, but this testing protocol is a lengthy process that does not meet the requirements of paint and stain producers who want faster evaluation of their new coating systems. Another limitation of the EN 927 test is the location of the exposure site, which narrows the variety of climatic effects which can be observed.

In order to overcome these limitations, artificial weathering tests (EN 927-6) were put into place. These determine the resistance of wood coatings to artificial weathering in an apparatus equipped with fluorescent UV lamps, condensation, and water spray devices. The trials are carried out in a Q-UV system which allows the test conditions to be fixed for a total exposure time of 2,016 hours.

Hexion recently engaged the CATAS Institute to test VeoVa™ 10 monomer-containing wood stains (as well as competitive systems) for performance and conformity to the EN 927-6 standard.

At the end of the exposure period, wood panels utilizing the various coating systems were assessed for flaking, chalking, cracking, gloss, color, general appearance and adhesion. The properties of the exposed coatings were also compared with those of unexposed coatings prepared from the same materials.

The composition and latex properties of the evaluated systems are as follows:

• 30% VeoVa: Core/shell polymer containing 30% of VeoVa 10 monomer
• 30% 2-EHA: Core/shell polymer containing 30% of 2-ethylhexylacrylate
• Acrylic Benchmark 1 (BM1): A commercial high-end acrylic emulsion polymer tailored towards the decorative wood coatings market
• Acrylic Benchmark 2 (BM2): A commercial high-end acrylic emulsion polymer tailored towards the industrial wood coatings market
• Solvent-based Alkyd (SB Alkyd): A commercial 2010 compliant high-solids solvent-based alkyd stain

Summary Discussion

After 2,016 hours of exposure, no sign of failure blistering, cracking or flaking was observed in any of the panels. Yet, the coating based on 2-EHA showed significant chalking. Gloss retention and color changes on the artificially aged panels are shown in figure 2 and 3. The positive effect of the VeoVa 10 monomer on the gloss retention of its coating system can be clearly observed¹. The gloss retention of the VeoVa-containing wood stain is superior to those of the two wood stains based on commercial binders and of the commercial solvent-based alkyd.

Weathering significantly alters the appearance of wood as well as its surface properties. Color changes (greying) in wood are due to the accumulation of photodegraded lignin constituents leached from the wood. The general appearance of the different coated panels is reflected in their color changes (ΔE*). The major change was in the 2-EHA-based wood stain, whereas the VeoVa 10-based wood stain shows the lowest change of color. Again, the effect of VeoVa 10 on color protection is significant. The combination of UV stability with increased hydrophobicity delivered by the VeoVa monomer incorporated into the polymer likely contributes to this improved appearance².

Conclusion

The wood stain based on the VeoVa/acrylic core/shell polymer significantly outperformed the stain based on the 2-EHA containing polymer and was superior to any of the other systems tested. The superior appearance of the VeoVa-based coating after artificial weathering is not only very visible but can also be clearly quantified by minimal color change and the excellent gloss retention of the formulated stain.

Figure 1. Accelerated weathering – General appearance (2016h EN927-6)

 

Figure 2. Accelerated weathering – Gloss retention (2016h EN927-6)

 

Figure 3. Accelerated weathering – Color changes (2016h EN927-6)

References:

1. a) Prior, R.A., Hinson, W.R., Smith, O.W., Bassett, D.R., Progress in Organic Coatings, 29, pp. 209-224, 1996. b) H. Hendrickx, N. Havaux, D. Vanaken, C. Nootens, F. Decocq, G. Badini, F. Simal, “High Performance Water Repellent Acrylic Lattices for Exterior Wood Coatings”, Proceedings of 3rd International Coating Wood and Wood Composites Conferences, 2009.
2. Decocq, F., Slinckx, M., Nootens, C., Journal of Protective Coatings & Linings, June, pp. 48-56, 2001.